This invention relates to a distributed-constant band-pass filter for use in a microwave communication system.
In general, a band-pass filter has a passband between two cuttoff frequencies and attenuation bands on both sides in a finite frequency band of the passband. For microwave communication, use is made as the band-pass filter of a distributed-constant filter having a plurality of resonator rods. It is desirable for the band-pass filter to have sufficiently large attenuation in the attenuation band and sharp cutoff edges. A conventional distributed-constant filter is often of a Butterworth or Chebyshev response characteristic and has no attenuation pole outside in the finite frequency band of the passband. Use is therefore inevitable of a band-reject filter in combination with the band-pass filter to accomplish the large attenuation and the sharp cutoff edges.
In "IEEE Transactions on Microwave Theory and Techniques" (June 1966), pp 295-296, R. M. Kurzrok proposed a microwave distributed-constant filter comprising two first resonator rods coupled to an input and an output terminal, two second resonator rods inductively coupled in series between the first resonator rods, and a coupling probe between the first resonator rods. The proposed bandpass filter is of a band-pass characteristic having two attenuation poles in the finite frequency band and consequently has sharp cutoff edges. Assembly of the filter is, however, complicated because the coupling probe has to be insulated from the first resonator rods. The filter is fragile against a mechanical shock. Moreover, the frequencies at which the attenuation poles appear are not adjustable because it is impossible to adjust the coupling probe that determines the attenuation pole frequencies.